Unfortunately, bookmarklets don't work on Firefox mobile. Fortunately however, the "Hide Fixed Elements" and "Scroll Everywhere" add-ons do work on mobile:
https://addons.mozilla.org/en-US/firefox/addon/hide-fixed-el...
https://addons.mozilla.org/en-US/firefox/addon/scroll-everyw...
That is, if you jump through the hoops to make more than the default 8 add-ons available on Firefox mobile:
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...